1. Field of the Invention
The present invention relates to a laser illuminating apparatus for irradiating a linear spatial light modulator.
2. Description of the Related Art
Such applications as computer-to-plate and computer-to-press require a high power laser source to expose heat-sensitive material. In such a case, a bar laser (i.e. a broad-area semiconductor laser) including a plurality of emitters arranged linearly for emitting numerous laser beams in which main beams are substantially parallel can be used.
An image recording system proposed heretofore includes such a bar laser and a spatial light modulator. Laser beams emitted from the bar laser are superimposed on the spatial light modulator through a microlens array having a plurality of microlenses arranged in a corresponding relationship to the emitters of the bar laser. The spatial light modulator divides each laser beam into a plurality of laser beams, and modulates these laser beams individually (see Japanese Patent Publication (Unexamined) No. 1997-230280 and U.S. Pat. No. 5,900,981, for example).
With an illuminating apparatus used in such an image recording system, the microlens array consists of a plurality of microlenses arranged in the arrangement of the emitters in the bar laser. This arrangement results in difficulties in adjusting the microlens array as well as increased manufacturing cost. Further, where such a construction is employed, the bar laser can include only around 20 emitters for reasons of manufacturing and adjustment of the microlens array. Thus, it is difficult to use a very high power (xe2x89xa760 w) bar laser with more than 20 emitters.
In addition, the illuminating apparatus used in the above image recording system has a construction for imaging near field patterns of the laser beams emitted from the bar laser on the spatial light modulator. The near field patterns being multi-mode, the consequent light distribution on the spatial light modulator tends to be nonuniform and unstable.
On the other hand, Japanese Patent Publication (Unexamined) No. 1998-278345 has proposed an image recording system constructed for superimposing far field patterns of laser beams emitted from a bar laser on a spatial light modulator.
The image recording system according to Japanese Patent Publication (Unexamined) No. 1998-278345 achieves a uniform light distribution. However, the light distribution is not uniform enough.
The object of the present invention, therefore, is to provide an illuminating apparatus easy to adjust and inexpensive to manufacture even with a bar laser having numerous emitters, and yet capable of irradiating a stable and uniform illumination area.
The above object is fulfilled, according to the present invention, by a laser illuminating apparatus for irradiating a linear spatial light modulator, comprising a bar laser including a plurality of linearly arranged emitters; a lens array consisting of a plurality of lens units less in number than the emitters and arranged parallel to the direction of arrangement of the emitters, for causing a collimated laser beam incident thereon to converge at the final surface of the lens array at least on a plane including the direction of arrangement of the emitters and the optical axis; and optics for creating a superimposed far field patterns, in the direction of arrangement of the emitters, of the laser beams emitted from the bar laser on the first plane of the lens array, projecting the far field patterns divided by the lens array onto the illumination area in superimposition, and projecting near field patterns, in the direction perpendicular to the direction of arrangement of the emitters, of the laser beams emitted from the bar laser on the illumination area in cooperation with the lens array.
In a preferred embodiment of the invention, the lens array comprises a cylindrical lens array, and the optics comprises:
a first cylindrical lens having power in the direction of arrangement of the emitters for creating a superimposed the far field pattern, in the direction of arrangement of the emitters, of the laser beams emitted from the bar laser to the first plane of the lens array, the emitters of the bar laser being disposed in the front focal position of the first cylindrical lens, the cylindrical lens array having a first surface thereof disposed in the back focal position of the first cylindrical lens;
a second cylindrical lens having power in the direction of arrangement of the emitters for projecting the far field patterns divided by the lens array onto the illumination area in superimposition, the cylindrical lens array having a final surface thereof disposed in the front focal position of the second cylindrical lens, and the illumination area being disposed in the back focal position of the second cylindrical lens; and
a pair of third cylindrical lenses each having power in the direction perpendicular to the direction of arrangement of the emitters for projecting the near field patterns, in the direction perpendicular to the direction of arrangement of the emitters, on the illumination area.
In another preferred embodiment, the lens array comprises a spherical drum lens array, and the optics comprises:
a first spherical lens for creating a superimposed far field pattern, in the direction of arrangement of the emitters, of the laser beams emitted from the bar laser to the first plane of the lens array, the emitters of the bar laser being disposed in the front focal position of the first spherical lens, the spherical drum lens array having a first surface thereof disposed in the back focal position of the first spherical lens;
a second spherical lens for projecting the far field patterns divided by the lens array onto the illumination area in superimposition, the spherical drum lens array having a final surface thereof disposed in the front focal position of the second spherical lens, and the illumination area being disposed in the back focal position of the second spherical lens; and
a plurality of cylindrical lenses each having power in the direction perpendicular to the direction of arrangement of the emitters and projecting the near field patterns in cooperation with the spherical drum lens array and the first and second spherical lenses, in the direction perpendicular to the direction of arrangement of the emitters, on the illumination area.